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# Motor-Porn: Honda GX25 moved into new APU rig

What started out as a simple thread on a mobile-home board, ended up in more than 50 pages of discussion about a neat and small generator project based on a Honda GX25 and a Torcman 430-30 brushless outrunner motor. Some people always seem to ask, why anyone would build something like that, if they could get more output power for less money from a ready made product? Because available products don't really match the requirements and why would anyone pay money for something one actually doesn't really want or need?

Apollo-NG needs multiple independent energy sources to be able to function autonomously and independently from grid infrastructure. Although the basic requirements are supposed to be covered by alternative power technologies like solar and wind power, it never hurts to have a fallback for a rainy day. That's where the Auxiliary Power Unit comes into play:

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However, it first started with this test setup to test the engine and the generator as a general prove-of-concept and to make sure that all parts were running satisfyingly, before taking them apart and reduce the whole thing to the max:

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Last week, the extruded aluminum profiles for the rig arrived and were assembled by iggy, who seemed to like to play with LEGO for adult engineers. It seemed pretty easy, but maybe it's better if he'd care to comment on the ease and quality of the build. After stripping down the engine and removing all parts that were deemed unwanted, a big pile of spare parts piled up:

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When the engine is in the rig it's pretty easy to get a good imagination how things are going to look, when it's finished:

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The next task is to reduce sound emissions by creating a new muffler system. Research showed, that a simple 4:2:3 three chamber ratio in a body 20 times the volume of the engine (displacement) should provide good results and should have no effect on the output power of the engine. The preliminary draft design on the right basically shows the setup with the three chambers and the input (upper) and output (lower) pipes in a 90° angle.

Luckily, Harald Zimmermann, who is somewhat of an expert on the topic, since he is designing and building mufflers for a long time, agreed to help out, by designing a muffler with good sound suppression and hopefully no noticeable engine power losses. Hopefully the APU will purr more quietly once the new muffler is ready and operational.

If you are planning to build this too and have trouble getting the GX25 engine for a good price start thinking out of the box: These engines, when bought as a single unit, are nowhere available for under 300EUR, but you might want to look for Honda UMS425 motor trimmers - which basically have the exact same engine, this one used to be an UMS425 too :)

## Discussion

, 2012/11/26 05:10

Fantastic. Almost the same idea I have. I'm building mine for 12V output. I'm quite interested in the muffler, and would like to see more of it. Brilliant

, 2012/11/26 14:31

Hey Jeff,

this one actually is designed for 12V output, since Apollo's Main-Power-Bus is going to be 12V (to be able to use off-the-shelf 12V AGM batteries and components). The idea here is to let the Motor/Generator work at their optimal operating range and use an efficient step-down converter with a wider input range to always get 12V out of the system, you might want to have a look at the Power Distribution Unit (PDU) and the basic concept of the UPC which will be a similar design, to get the most out of this system and can also protect the components/batteries from over-voltage/over-current situations.

As for the muffler, the parts are already here, but at the moment unfortunately there is no time left to continue the development because I was forced to take up another dayjob since there weren't enough people supporting the project and I have to get the money for the next development steps myself. However, the price to pay, is the time I have to waste on the dayjob now, instead of making and documenting the next steps. I hope that I will be able to continue with the development rather soon, so thank your for the feedback, that motivates me a lot to get on with it again :)

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